In continuing experiments, we have identified phases and lesions which appear to represent obligatory alterations in the process of hepatocarcinogenesis, induced by agents of differing chemical classes. Our proposed studies will examine several aspects of the interaction of these agents with DNA at these points. In brief, these will include: 1. Further examination of our finding that a DNA polymerase-alpha which is present during exposure to acetylaminofluorene, demonstrates significant infidelity in replicating DNA. This study will include analysis of the nature of the alterations, its persistence, presence or absence during other carcinogenic regimen, and the interaction of this enzyme with altered templates. 2. Expansion of our studies of the composition and structure of DNA isolated from the induced hepatic lesions and the malignant tumors which result. For this purpose, we will attempt to increase the range of DNA subfractions which we have resolved with restriction endonucleases; further develop our analysis of alterations of DNA methylation in these fragments; and, develop and apply our technique of nucleotide resolution and analysis to an examination of base-adducts resulting from exposure to carcinogens. 3. We will continue to analyze the role of the nuclear membrane in carcinogen activation and DNA binding. Extension of an examination of the biologic alterations of these lesions will continue.